This invention relates to multilayer ceramic capacitors and formulated ceramic powders for making the capacitors that meet X7R performance characteristics of the Electronic Industrial Association (xe2x80x9cE.I.A.xe2x80x9d), and in particular relates to a formulated ceramic powder from which there can be made a dielectric ceramic body that can be sintered at or below 1,025xc2x0 C. to permit the usage of a high silver content silver-palladium alloy electrode having an 85 weight percent or greater silver composition.
Efforts to produce low fired, high dielectric constant, X7R multilayer ceramic capacitors and formulated ceramic powder for making the capacitors are well known. For example, dielectric ceramic compositions that lead to a mature dielectric body with standard X7R performance characteristics and a high dielectric constant afer firing or sintering at about 1,100xc2x0 C. are disclosed in several patents by one of the present inventors, Galeb Maher. Those patents include U.S. Pat. No. 5,010,443 that issued on Apr. 23, 1991 to Maher, U.S. Pat. No. 5,258,338 that issued on Nov. 2, 1993 to Maher, and U.S. Pat. No. 6,043,174 that issued on Mar. 28, 2000 to Maher et al., all of which patents are hereby incorporated herein by reference and all of which are assigned to the same assignee that is also the assignee of all rights in the present invention disclosed and claimed herein.
In the aforesaid U.S. Pat. No. 6,043,174 to Maher et al., a highly acceptable X7R formulated ceramic powder mixture is described that may be sintered after formulation at a temperature as low as 1,120xc2x0 C. in order to permit usage of electrodes consisting of 70% silver and 30% palladium. The ceramic powder mixture disclosed in the 6,043,174 Patent comprises at least ninety weight percent (xe2x80x9cwt %xe2x80x9d) pure barium titanate powder having an average particle size of from 0.4 to 0.7 microns, from 1.5 to 2.5 wt % of a cadmium silicate powder flux, a small amount of a grain growth inhibitor compound, such as 0.89 to 2.72 mole percent Nb2O5, and also about 0.2 to 1.0 mole percent calcium cations relative to the amount of barium titanate. Multilayer ceramic capacitors made from such a composition are currently being made and sold in the industry, and formulated ceramic powders made according to the disclosures in U.S. Pat. No. 6,043,174 are being sold by the assignee of the present invention, MRA Laboratories, Inc., of Adams, Mass., U.S.A.
Such ceramic powders and multilayer ceramic capacitors made from those powders have achieved high performance characteristics while reducing an amount of palladium within an electrode. As is well known, the high cost of palladium has made it desirable to decrease an amount of palladium and to increase an amount of silver in a silver-palladium alloy electrode. By decreasing a sintering temperature at which the ceramic powder is converted from a powder into a cohesive solid or mature body without melting the powder, the relative amount of silver in the silver-palladium electrode may be increased while the amount of palladium may be decreased. As indicated above, the ceramic powders disclosed in the 6,043,174 Patent enabled sintering at a temperature of between 1,120xc2x0 C. to 1,150xc2x0 C. so that the amount of palladium in the electrodes could be reduced to 30 percent. It is understood in the art that the sintering temperature of the ceramic powder must be below the melting temperature of the silver-palladium electrode. Additionally, as the proportion of silver in the electrode is increased and the proportion of palladium in the electrode is decreased, the melting temperature of the electrode is decreased. Therefore, by lowering the sintering temperature of the ceramic powder, a greater proportion of silver and smaller proportion of palladium may be utilized in making the electrode to effect a significant cost saving due to the substantially higher cost of palladium compared to the cost of silver.
The aforesaid Patents also disclose further research efforts of the inventors herein to decrease costs and enhance performance characteristics of formulated ceramic powders for multilayer capacitors. For example, U.S. Pat. No. 6,043,174 also discloses that by addition of a very small and critical quantity of calcium compound such as calcium carbonate or calcium niobate to the start powder, a costly anneal step may be eliminated while maintaining a high insulation resistance in an accelerated life test with only a very small reduction of dielectric constant.
While known formulated ceramic powders and multilayer capacitors made from the powders demonstrate improved performance, nonetheless existing powders still suffer from significant limitations. For example, it is known to use heavy metal oxides such as CdO, PbO and Bi2O3 in the powders as a flux. The advantages of use of cadmium silicate flux in multilayer capacitors were first disclosed by one of the inventors herein in U.S. Pat. No. 4,266,265 that issued on May 5, 1981 to Galeb Maher. However, such heavy metal compounds as cadmium oxide are increasingly considered hazardous materials, and hence they pose significant cost problems related to usage and disposal of components utilizing capacitors that include cadmium or other hazardous heavy metal oxides. Additionally, as disclosed in the aforesaid patents, in sintering ceramic powders including a cadmium silicate flux, a closed crucible must be used to contain any toxic cadmium fumes from contaminating persons in the vicinity of the crucible.
Accordingly, there is a need for a multilayer ceramic capacitor and powder for making the capacitor that does not include hazardous heavy metal oxides, and that reduces costs of known capacitors.
The invention includes a dielectric ceramic powder mixture comprising at least ninety weight percent essentially pure barium titanate powder having an average particle size of from 0.2 to 1.2 microns; from 0.2 to 2.5 weight percent of barium lithium borosilicate (xe2x80x9cBLBSxe2x80x9d) flux; from 0.1 to 0.3 weight percent of MnCO3; a grain growth inhibitor such as niobium oxide or a niobate compound so that when added to the powder mixture the niobium oxide or niobate compound yield between 0.4 to 1.50 weight percent Nb2O5; and, 0.4 to 1.2 weight percent of Gd2O3.
As a result of exhaustive research and experimentation, it was discovered by the inventors that the rare earth oxide of gadolinium produced unexpected and highly desirable results in yielding a fine grain structure, and that when combined with Nb2O5 or a niobate compound as a grain growth inhibitor, a desirable core-shell structure known in the art is obtained that demonstrates excellent X7R performance characteristics after firing or sintering at as low a temperature as 950xc2x0 C.
An additional aspect of the invention includes a method of making the preferred dielectric ceramic powder, which includes the steps of preparing the above described mixture of start powders including the 0.4 to 1.2 weight percent of Gd2O3; mildly calcining the start powder mixture at about 450xc2x0 C. to about 550xc2x0 C. to obtain a powder comprised of agglomerates of the mixture of start powders wherein each of the agglomerates has essentially the same compositional ratios as the start powder of the barium titanate, barium lithium borosilicate, Nb2O5, MnCO3, and gadolinium oxide (Gd2O3); in order to provide a powdered mixture capable of being sintered in an open zirconia setter at from 950xc2x0 C. to 1,025xc2x0 C. to produce a dielectric ceramic body yielding a fired density greater than 5.75 g/cm3 a dielectric constant (xe2x80x9cKxe2x80x9d) greater than 2,500, and a smooth temperature coefficient of capacitance (xe2x80x9cTCCxe2x80x9d) wherein the K will be within +/xe2x88x9215% relative to 25xc2x0 C. temperature values within a temperature range of between xe2x88x9255xc2x0 C. to 125xc2x0 C.
Another aspect of this invention includes a method for making a multilayer ceramic capacitor body that satisfies X7R performance characteristics that includes the steps of forming a slurry of the above described calcined start powder mixture including the 0.4 to 1.2 weight percent of Gd2O3, by a dispersion of the mixture in a binder-solvent-dispersion system; preparing layers of the slurry; drying the layers; forming a stack of a plurality of the layers and interleaving patterned films of silver-palladium electrode ink between successive adjacent pairs of layers; and then heating to mature the stack of layers by sintering in open air at a temperature of between 950xc2x0 C. to 1,0250xc2x0 C. for between 3 to 5 hours to produce a dense ceramic capacitor body with buried electrodes.
The multilayer ceramic capacitor made by that method demonstrates the aforesaid favorable X7R performance characteristics of K greater than 2,500, and a smooth temperature coefficient of capacitance (xe2x80x9cTCCxe2x80x9d) wherein the K will be within +/xe2x88x9215% between xe2x88x9255xc2x0 C. to 125xc2x0 C., a dissipation factor (xe2x80x9cDFxe2x80x9d) of less than 2.5 percent, an insulation resistance greater than 100 ohms-farads at 125xc2x0 C., and long term stability during a life test at elevated temperatures and voltages.
It is therefore an object of this invention to provide a very low fired, high dielectric constant ceramic capacitor and the powder for making the capacitor that includes a gadolinium oxide additive and includes no heavy metal oxides. The resulting powder and ceramic capacitor has smooth X7R performance characteristics, may be fired or sintered at temperatures between 950xc2x0 C. to 1,025xc2x0 C., and includes no hazardous waste products thereby minimizing manufacture and disposal costs, while permitting usage of a high silver content silver-palladium electrode having a sliver content in excess of 85 percent.
These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the a accompanying drawings.